The present invention relates to a compression method for (digital) moving images that have been interlaced, and more particularly, relates to a favorable predictive coding method for image signals in media of storage systems.
The motion compensation inter-frame predictive coding method shown in FIG. 1 is known as an effective method for the compression of moving image (signals). This method detects an amount of motion (motion vector) between an input image and a comparison image, and subtracts from the input image the prediction image that has been shifted comparison image by the motion vector and codes this error value and the motion vector.
The motion vector is generally detected using the block matching method. In this method, an image for processing is desirably a frame image which is not interlaced. If there is the detection of a motion vector with respect to a frame image that is still interlaced, then the influence of a time difference between two fields and resulting from the interlacing increases the high-frequency component in the vertical direction of the frame, and reduces the correlation in the vertical direction, between the input image and the comparison image. As a result, the accuracy decreases with respect to the detected motion vector, thereby increasing the error value between the input image and the prediction image.
Here, when the image for processing is an interlace image, it is known that the method of generating a non-interlaced frame from two fields, and detecting the motion vector with respect to that frame and performing predictive coding prevents deterioration of the accuracy of the motion vector.
The methods for the generation of a frame image that is not interlaced, are a method where a frame is generated by simply interpolating one field, and as shown in FIG. 2, a method that judges a motion region and a still region from both fields, and generates an application frame by pasting both fields in the still region and inserting either one field into the motion region.
In the method where a frame is generated by interpolating one field, the resolution in the vertical direction of the generated frame deteriorates by half and the quality of the image drops. In addition, in the method where the motion region and the still region are judged and a frame is generated applicably, the correlation in the vertical direction of the frame is reduced because of the insertion of unsuitable picture elements when there is not the accurate extraction of the still and motion regions between both fields, the accuracy of the motion vector greatly decreases and the prediction efficiency deteriorates.
Furthermore, in order to avoid obstruction in interlaced images such as this, there has been proposed the method shown in FIG. 3, where the two fields of odd-numbered fields and even-numbered fields are handled completely separately. In this method, there is no deterioration of the degree of resolution in the vertical direction and thus there is no deterioration of the accuracy of the motion vector, but there is the disadvantage that the separate processing for the fields prevents the use of the correlation between the respective fields.
By the way, when there is the recording and reproduction of image information that has undergone predictive coding, to and from storage media, it is necessary to provide image signals that become a reference at required intervals so that there can be the effective functioning of random access and search and the like.
With respect to non-interlaced image signals, the MPEG (Moving Picture Expert Group) has proposed standards for which standardization surveys were conducted in ISO IEC JTC1/SC2/WG8 as a method for inter-frame predictive coding for media of storage systems.
FIG. 4 is a view describing the coding sequence of the MPEG standards proposal.
This MPEG standards proposal sets a frame I that becomes the reference, at each required interval of frames, and this reference frame I is used as the basis for the setting of sub-reference frames for every M number of frames (where M is a natural number).
The reference frame I performs intra-frame coding.
The sub-reference frame P performs inter frame prediction in the uni or one direction from the previous reference frame I or the previous sub-reference frame P.
The other frame B performs frame prediction in both directions from the P frames or the I frames on both sides.
FIG. 5 is a block diagram of a coding apparatus of the MPEG standards proposal.
This coding apparatus 1 comprises a frame memory 3 that stores digital image signals 2a that are non-interlaced and which are supplied to an input terminal 2, a subtraction means 4 that generates prediction error signals 4a, a discrete cosine transformation circuit 5 that implements discrete cosine transformation, with respect to the prediction error signals 4a and outputs a transformation coefficient, a quantizer 6 that uses a coefficient having a required weighting to quantize a transformation coefficient 5a, a variable length coder 7 that codes a motion vector along with a quantized output 6a as well as information 9a relating to the calculation conditions, a local decoding means 8, an interframe motion compensation circuit 9, a motion vector detection circuit 10 and frame memories 11 and 12 for the forward and after directions.
The local decoding means 8 is provided with an adding means 15, an inverse discrete cosine transformation circuit 14 and a inverse quantizer 13 so as to decode the predictive error signals 4a.
The input digital image signals 2a are stored in the frame memory 3.
The image signals of the reference frame I are input as they are to the discrete cosine transformation circuit 5, undergo discrete cosine transformation, and the respective coefficients 5a that are transformed are quantized by the quantizer 6, are coded by the variable length coder 7 and are output as the coded data 7a.
The quantized output 6a is decoded by the local decoding means 8. These decoded image signals 8a are stored in the forward direction frame memory 11 or the after direction frame memory 12.
On the other hand, when there are image signals of a sub-reference frame P and another frame B, the motion vector detection circuit 10 determines the motion vector 10a between image signals of this frame and the decoded image signals 11a and 12a that are stored in the forward direction frame memory 11 and the after direction frame memory 12.
The motion vector 10a is detected using the block matching method for example, but some other method can be used.
The frame image for which the motion vector 10a has been determined has inter-frame motion compensation prediction performed by the inter-frame motion compensation prediction circuit 9, and the motion compensation prediction image signals 9b are generated.
When there is a sub-reference frame P, the image signals of the frame I (or P) that are stored in the forward direction frame memory 11 are used as the basis for the performance of motion compensation prediction in the forward direction.
When there is another frame B, the two types of frame image signals that are stored in the forward direction frame memory 11 and the backward direction frame memory 12 are used as the basis for motion compensation prediction for the three types of prediction methods of forward direction, backward direction, and forward and backward direction interpolation prediction, and the optimum prediction method is selected.
The prediction error signals 4a between the motion compensation prediction image signals 9b that are output from the inter-frame motion compensation prediction circuit 9 and the image signals of each of the input frames P and I, are processed by the discrete cosine transformation circuit 5, the quantizer 6 and the variable length coder 7 in the same manner as for the reference frame I described earlier, and the coded data 7a is output.
Moreover, with respect to a sub-reference frame P, the prediction image 9b and the output signals of the inverse cosine transformation circuit 14 are added by the adder circuit 15 and the configuration is such that the decoded image signals 8a that are obtained are stored in the forward direction frame memory 11 or the after direction frame memory 12.
However, the previously described MPEG standards proposal has non-interlaced image signals as its object and so there are the problems that were decribed earlier when such a coding method is used for the processing of interlaced image signals.
First, when each field of an interlace image is overlapped and handled as a frame, the influence of the time difference between two fields increases the high-frequency component in the vertical direction of the frame and reduces the coding efficiency by intra-frame coding.
In addition, in inter-frame prediction coding, the correlation in the vertical direction decreases in the same manner and even if motion compensation is performed, there are different amounts of motion between frames of two fields when there is zooming and the like and so the prediction efficiency decreases greatly to give an adverse influence on the image.
When the method shown in FIG. 3 is used with respect to inter-frame prediction, there is coding of prediction values between frames for both pairs of odd-numbered fields and pairs of even-numbered fields and two fields in a frame image for which intra-frame coding has been performed, have individual intra-field coding respectively performed for them.
In this case, there is no deterioration of the degree of resolution in the vertical direction of the frame, and there is no loss of the prediction accuracy between frames but since there is separate processing for the fields, it is not possible to use the correlation between the respective fields and in particular, there is the disadvantage that there is an increase in the amount of information in the intra-frame coding.